Declaration of Head/First node in a Linked list - c++

Question

Hi all I'm having an issues in a linked list problem. Given two piece of code I've to find why one of them is not working
Code 1 is
struct node {
int data;
struct node *link;
};
void insert(struct node *head) {
struct node *last, *temp;
head = (struct node *)malloc(sizeof(struct node));
printf("Input an integer: ");
scanf("%d", &head->data);
head->link = NULL;
last = head;
{
int n = 3;
while(n>0){
temp = (struct node *)malloc(sizeof(struct node));
printf("Input an integer: ");
scanf("%d", &temp->data);
temp->link = NULL;
last->link = temp;
last = temp;
n--;
}
}
return;
}
void display(struct node *p) {
while(p) {
printf("%d ",p->data);
p = p->link;
}
return;
}
int main() {
struct node *head;
insert(head);
display(head);
return 0;
}
and second code is
struct node {
int data;
struct node *link;
}*head;
void insert() {
struct node *last, *temp;
head = (struct node *)malloc(sizeof(struct node));
printf("Input an integer: ");
scanf("%d", &head->data);
head->link = NULL;
last = head;
{
int n = 3;
while(n>0){
temp = (struct node *)malloc(sizeof(struct node));
printf("Input an integer: ");
scanf("%d", &temp->data);
temp->link = NULL;
last->link = temp;
last = temp;
n--;
}
}
return;
}
void display(struct node *p) {
while(p) {
printf("%d ",p->data);
p = p->link;
}
return;
}
int main() {
insert();
display(head);
return 0;
}
Now my question is why declaring head in main in the first is not giving o/p for display function wheres declaring it globally in second code is working? Asking this as I'm wondering that in first case head is declared in main and passed as an address so after coming back from insert function it should get the effect of that insert function operation but it's not working like the way and not giving ant o/p for display function

Answer 1

The issue is that, in the first code, insert receives a copy of the main's head pointer and modifies that copy by making it point to some newly allocated memory. That modification never propagates back to main.
To make it propagate, use a pointer to pointer:
void insert(struct node **head) {
struct node *last, *temp;
*head = (struct node *)malloc(sizeof(struct node));
printf("Input an integer: ");
scanf("%d", &(*head)->data);
(*head)->link = NULL;
last = *head;
{
int n = 3;
while(n>0){
temp = (struct node *)malloc(sizeof(struct node));
printf("Input an integer: ");
scanf("%d", &temp->data);
temp->link = NULL;
last->link = temp;
last = temp;
n--;
}
}
return;
}
and then, in main, call it like so:
insert(&head);
Alternatively, you could make insert take a pointer but also return a pointer (i.e. the new head):
struct node* insert(struct node *head) { ... }
One issue what that API is that it's rather error-prone: it's very easy to call insert() and forget to deal with its return value.